CaV1.3 channels are low-threshold, dihydropyridine-sensitive L-type Ca2+ channels which mediate low-voltage signaling and rhythmicity throughout the body. They are essential for neurotransmitter release at ribbon synapses such as found in cochlear hair cells; they mediate pacemaking in the heart; and they modulate oscillatory behavior throughout the brain, such as the repetitive bursting in supra-chiasmatic (circadian pacemaking circuitry) and substantia nigra (locus of primary damage in Parkinson's) nuclei. As such, overactivity of these channels may predispose for Ca2+ overload precipitating Parkinson's, and downward modulation of these channels may enhance positive mood and affect. Clearly, small-molecule compounds that selectively inhibit or enhance CaV1.3 channels, rather than the other CaV1 L-type channels would be of enormous utility for basic studies of CaV1.3 roles, and for potentially amerliorating a number of CaV1.3-related pathologies. However, though excellent L-type channels antagonists and agonists have been discovered, none can truly select among the L-type channel subtypes. Here, in the search for selective modulators, we will exploit a unique molecular interaction between ICDI and IQ domains of CaV1.3 channels, where this interaction modulates the strength Ca2+ feedback inhibition (CDI) of these channels. This promising screen will be prosecuted according to three specific aims. 1) To perform a primary screen for small molecules that disrupt or enhance a functionally critical interaction between IQ and ICDI domains of CaV1.3 channels, using the MLSMR library of 350,000-500,00 compounds. 2) To confirm and identify candidate hits from Aim 1 using a microscope-based FRET analysis of single living cells. 3) To test candidate compounds for modulation of CaV1.3 Ca2+ regulation, using patch-clamp electrophysiology. Overall, this project promises lead candidates for selective modulators of CaV1.3 versus other CaV1 L-type calcium channels.